1. Field of the Invention
The present invention relates to the field of powder coating, the application of powders as coatings onto surfaces, and apparatus used for the application of powders onto other surfaces by pneumatic or spray methods.
2. Background of the Art
In powder coating installations, the powder coating material has been usually pneumatically conveyed by the dilute phase method from a reservoir to a spray gun through a hose-like delivery line. Relatively large amounts of compressed air are needed to do this. Where filtering is performed on powders, the powders may tend to agglomerate, clog the filters, reduce the controlled rate of powder flow, and otherwise interfere with the coating operation.
Plug flow conveying or dense phase conveying has been used in which the powder coating material is cyclically conveyed either by gravity or by negative pressure into a chamber and then discharged from the chamber with compressed air and conveyed in the form of successive “plugs” through the delivery line to the spray application device. This provides a less desirable and non-steady state flow of materials. A device of the type is shown in EP 0 937 004 B1. This device has a cylindrical pump chamber, which is equipped with a discharge opening for the conveyed material at its lower end and a plate-like filter element that is impenetrable by the conveyed material at its upper end, by which the pump chamber can be alternately connected with a vacuum pump and with a source of compressed gas, to suck the conveyed material from a reservoir through a connection that opens into the pump chamber from the side for the purpose of filling the pump chamber, or to push the conveyed material through the discharge opening into a discharge line for the purpose of emptying the pump chamber. To allow exact metering of the conveyed material and at the same time a high discharge volume, the pump chamber should have a filling volume that is as small as possible, and it should be possible to fill it and empty it in an operating cycle that is as short as possible. To achieve the latter objective, however, the gas must be sucked from the pump chamber and fed into the pump chamber relatively quickly, which requires a pressure difference that is as large as possible between the inside of the pump chamber and the source of negative pressure or the source of compressed gas.
A more typical spray application system for powder coating comprises a powder reservoir over which a flow of air is directed, picking up powder from within the reservoir and directing it against a surface to be coated. Often a resource pipe or tube extends from the flow of air into the reservoir is used to collect and carry powder (often in a fluidized bed state) into the flow of air by Bernoulli effects. The use of filters to limit size ranges of particles is important, but again suffers from attendant problems of aggregation, filter clogging and the like.
U.S. Pat. No. 7,543,764 (Haynes) describes a nozzle for use in a cold spray technique is described. The nozzle has a passageway for spraying a powder material, the passageway having a converging section and a diverging section, and at least the diverging section being formed from polybenzimidazole. In one embodiment of the nozzle, the converging section is also formed from polybenzimidazole.
U.S. Pat. Nos. 7,481,605 and 7,150,585 (Kleineidam) describes a process and equipment for conveyance of powdered metal, especially during powder coating processes. A gas-permeable filter element is designed as a hollow cylinder in such a way that it forms a part of the cylindrical surface of the conveyance chamber. The idea is to increase the filter surface of the filter element and reducing the pressure difference between the outer and inner surface of the filter element at the same gas throughput, without increasing the volume of the chamber and thus adversely affecting the metering precision.
U.S. Pat. No. 7,163,359 (Moser) describes a transport system with several devices (1) for transporting powder, where each device (1) has a transfer chamber (3) into which a supply line (6) and a discharge line (8) for the powder open, and means to generate a negative pressure in the transfer chamber (3), where the means to generate a negative pressure in the device (1) has a piston (11) which is moveable in the transfer chamber.